Process for purifying a gas containing hydrogen sulfide with production of sulfur

ABSTRACT

A process for purifying a gas containing hydrogen sulfide as the impurity, with the production of sulfur as a by-product which comprises contacting a gas containing hydrogen sulfide with a liquid phase containing at least one phosphoric acid ester and at least one alkanolamine, separating the purified gas free of hydrogen sulfide and treating the resulting solution containing hydrogen sulfide with a molecular oxygen-containing gas and separating sulfur therefrom.

United States Patent Renault et al.

1 51 Apr. 25, 1972 [54] PROCESS FOR PURIFYING A GAS CONTAINING HYDROGENSULFIDE WITH PRODUCTION OF SULFUR [72] Inventors: Philippe Renault,Neuilly-sur-Seine; Henri Gruhler, Chatillon sur Bagneux, both of France[73] Assignee: Institut Fruncals Du Petrole Des Carburants EtLubrifiants, Rueil-Malmaisen, Hants de Seine, France [22] Filed: Sept.5, 1968 211 Appl. No.: 757,736

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[56] References Cited 'UNITEDSTATES PATENTS 2,760,848 8/1956 Dunning..23/22s 2,987,379 6/1961 Urban ..23/22s 3,441,379 4/1969 Renault....23/226 3,104,951 9/1963 Urban ....23/225 Primary Examiner-Earl C.Thomas Assistant Examiner-G. O. Peters Attorney-Craig, Antonelli & Hill[5 7] ABSTRACT ing gas and separating sulfur therefrom.

8 Claims, 1 Drawing Figure Patented April 25, 1972 INVENTORS Phi/i,Reyna/t 1' MM! gnufiiul ATTORNEYS PROCESS FOR PURIFYING A GAS CONTAININGHYDROGEN SULFIDE WITI-I PRODUCTION OF SULFUR The applicant haspreviously described in the French Pat. No. l,492,797, filed Sept. 18,1965, a process for converting hydrogen sulfide to sulfur by oxidationwith air or oxygen, using solvents of H 8 which are soluble in water,for example glycol ethers, containing small amounts of organic basesacting as catalyst. The conversion of H 8 to sulfur takes place with theproduction of water. One part of this water is carried along by the airwhich is used as oxidizer. The other part accumulates in the solvent,which results into a dilution of the latter. The presence of substantialamounts of water in the solvent tends to improve the formation of byproducts, usually oxidized derivatives of sulfur, for example sulfates,thiosulfates, thus reducing the desired conversion to elemental sulfur.It is thus necessary to eliminate the formed water in order to obtainthe desired conversion with the optimal yield.

It has now been discovered that other solvents than those describedabove could be used to convert H s to sulfur by means of anoxygen-containing gas such as air, in the presence of alkanolamines,these solvents exhibiting the particular advantage of beingsubstantially insoluble in water.

The solvents to be used according to this invention are the phosphoricesters of the general formula PO (OR);, in which the R radicals whichmay be identical or different, are selected amongst the monovalenthydrocarbon radicals, each containing at least three carbon atoms (forexample three to 20) particularly the alkyl, cycloalkyl and arylradicals, and the radicals of formula -(R, R where R is a hydrocarbonrest of two or three carbon atoms, R is a hydrogen atom or hydrocarbonrest having preferably one to five carbon atoms, and n is an integersuch as 1, 2 or 3.

By way of non-limitative examples, there will be mentioned the tributylester of orthophosphoric acid, the triisobutyl ester of orthophosphoricacid, the tricresyl esters of orthophosphoric acid, the tricyclohexylester of orthophosphoric acid, monocyclohexyl di-n.propyl orthophosphateand tri(butoxy -2 ethyl) phosphate.

The alkanolamines which may be used as catalysts are those proposed inthe French Pat. No. 1492797, and more particularly those of formulawhere at least one of the R groups is a group of formula C H OH where pis an integer from 2 to 6, the other groups being selected from thehydrogen atom, the alkyl groups of l to 18 carbon atoms and the C, H OHgroup where r is an integer from 2 to 6. The C, H and C, H groups mayconrain an internal 0 bond between two carbon atoms.

Alltanolamines which are highly soluble in the phosphoric esters willbe, however, preferred, for example N-methyl diethanolamines. Otheruseful amines are diethanolamine, diisopropanolamine, monethanolamine,di- 11. propanolamine, triethanolamine, N- butyl diethanolamine, N-methyl diisopropanolamine and aminoethoxyethanol.

The concentration of alkanolamine will be dependent of the amount ofhydrogen sulfide to be converted and the operating conditions. It mayvary, for example, from 0.1 to 50 percent by weight and will be mostoften of about 10 percent by weight with respect to the mixture ofphosphoric ester with alkanolamine.

The mixture may further contain small amounts of water, however lessthan 10 percent by weight, according to the components of the mixture,an excess of water resulting into the formation of a second liquid phasewhich may be easily removed by decantation. The mixture may also containa small amount of a mineral base. Finally diluents of known type,preferably insoluble in water, may be also used.

The joint drawing illustrates an embodiment of this invention.

In a first column 1, the treated gas is introduced through pipe 2 andhydrogen sulfide contained therein is absorbed into the mixture ofphosphoric ester and amine which is introduced through pipe 3. The esteris by itself a good solvent for hydrogen sulfide; however the absorptionis improved when the process is carried out under increased pressure.The used amine improves the purification of the gas, particularly whenthe partial pressure of hydrogen sulfide is-or tends to be low.

The temperature will be chosen as low as possible, for example in therange of 20 to 50 C., since the absorption goes together with a heatrelease; however, the invention also covers the use of temperatures of 0to 100 C. The pressure is usually chosen between 1 and 100 atmospheres.

The gas which is substantially free of H 8 is removed through pipe 4whereas the solution which issues at the basis of 1 through pipe 5 isusually subjected to a pressure release in 6 and then introduced intoanother column 7 where hydrogen sulfide is oxidized to sulfur by meansof bubbling oxygen or air, said gas being introduced through pipe 8.This constitutes the regeneration step for the mixture of ester withamine.

The amount of injected air is usually in excess, for example 2 to 10times and preferably 3 to 5 times that corresponding to thestoechiometrical amount for the conversion of H 8 to sulfur. The excessof air is removed through pipe 9. The regeneration column is maintainedat a temperature usually between 0 and 100 C., particularly between 20and 100 C., and preferably between 50 and 60 C. The pressures may beselected in the same range as that given for the first step of theprecess. The crystals of sulfur which form therein accumulate by gravityat the bottom of this column, as well as water resulting from thereaction. Sulfur is removed through pipe 10. It may be thereafter meltedor washed.

The solution of amine in the phosphoric ester which has thus beenregenerated by oxidation with air, may be reintroduced into column 1through pipe 3.

Usually two phases appear at the basis of the regeneration column 7. Alower aqueous phase with sulfur and a higher organic phase consisting ofthe mixture of ester with amine, this mixture being recycled, as shownbefore, to the absorption column through pipe 3. This is particularlythe case when in the regeneration column 7, the mixture of amine withphosphoric ester is introduced from the top through pipe 5 and thismixture flows down through the column.

However it is also possible to invert the connection points of pipes 3and 5 in the case, for example, where the regeneration column should befull of liquid.

The mixture containing absorbed H 8 should be then introduced at thelower part of column 7 whereas the same regenerated mixture should berecovered at the top of this column to be recycled to the absorptionstep.

If the impure gas also contains hydrocarbons, these may be recovered asa gaseous phase in pipe 11, following the pres sure release in unit 6.

The absorption and oxidation steps may also be carried out in one columnsuch as 1, when the dilution of the desulfurized gas by the unconvertedoxidizing gas is not troublesome.

This process may particularly be applied to gases which contain from 0.5to 100 percent by volume of hydrogen sulfide.

The following, non-limitative examples, illustrate this invention.

EXAMPLE 1 The treated gas consists of 2 percent by volume of H 8, 6percent by volume of carbon dioxide and 92 percent by volume of methane.This gas is introduced under a pressure of 40 lrg./cm. and at a rate of1.000 liters per hour at the bottom of a packed tower at the top ofwhich there is injected 2 liters per hour of liquid mixture of about 10percent by weight of methyldiethanol-amine with percent oftriisobutylphosphate. The resulting gas contains only 500 parts permillion of H 8. The resulting liquid mixture, containing H 8, istransferred at a rate of 2 liters per hour into the regeneration columnmaintained at 35 C. under the atmospheric pressure. At the bottom ofthis column, air is injected at a rate of 200 liters per hour. The yieldof sulfur with respect to H 5 subjected to oxidation is 97 percent. Thisyield remained unchanged after 20 hours of run.

After 8 hours, an aqueous phase formed which was separated by meredecantation without any trouble for the operation.

EXAMPLE 2 This example is given by way of comparison and forms no partof this invention,

Example I is repeated, except that there is used the monoethyl ester oftriethyleneglycol instead of triisobutylphosphate, in same amount byweight. The purification rate of the gas is substantially the same as inExample 1.

The amount of water in the solvent which is initially 0.5 percent byweight increases to percent after hours. During the same period theyield of sulfur falls from the initial value of 97 percent to the finalvalue of 88 percent.

It is thus clear that the increase of the concentration of water in thesolvent results into an increase of the secondary reactions. It is thusnecessary to have a supplemental step for regenerating the solvent, saidstep consisting, for example, to eliminate water by distillation whichincreases the total cost of the operation.

EXAMPLE 3 This example illustrates an embodiment of the process,according to which the process is conducted in one column.

There is treated a gas (essentially air) comprising 2.5 percent (byvolume) of H 5, 4 percent of CO 19 percent of 0 and 74.5 percent of NThis gas is caused to bubble at a rate of 500 liters per hour into asolution at 1 percent by weight of N-methyl diethanolamine intributylphosphate.

l-l S is practically completely converted (i.e., the gaseous effluent ofthe column contains substantially no H 5) and the yield of sulfur is 97percent with respect to H 8 introduced into the column.

EXAMPLE 4 Example 3 is repeated, except that methyldiethanolamine isreplaced by the same weight of diethanolamine.

Substantially same results are obtained.

EXAMPLE 5 Example 3 is repeated, using as solvent a solution of lpercent by weight diethanolamine in tri (butoxy -2 ethyl) phosphate.

The resulting gas contains 50 parts per million by volume of H 8 and theyield of sulfur is 95 percent with respect to oxidized H 8.

What is claimed is:

l. A process for purifying a gas containing hydrogen sulfide as theimpurity, with the production of sulfur as a by-product which comprisescontacting a gas containing hydrogen sulfide with a liquid phasecontaining at least one phosphoric acid ester and at least onealkanolamine, separating the purified gas free of hydrogen sulfide andtreating the resulting solution containing hydrogen sulfide with amolecular oxygen-containing gas and separating sulfur therefrom; saidphosphoric acid ester-alkano'lamirie treatment and the oxidizingtreatment being conducted at a temperature of from about 0 to C.

2. A process according to claim 1, wherein the reaction is carried outin two steps wherein the first one consists of contacting the impure gaswith the mixture of an alkanolamine and a phosphoric acid ester, and thesecond one consists of treating the resulting solution, separated fromthe purified gas,

with the molecular oxygen-containing gas.

3. A process according to claim 1, wherein the alkanolamine is used inan amount of about 0.1 to 50 percent with respect to the weight of themixture of the phosphoric acid ester and the alkanolamine.

4. A process according to claim 1, wherein the step of contacting thegas with phosphoric acid ester and the step of oxidizing the resultingsolution are carried out simultaneously.

5. The process of claim 1, wherein the phosphoric acid ester is of thegeneral formula PO(OR) wherein R is selected from the group consistingof a monovalent hydrocarbon radical containing about three to 20 carbonatoms and a radical of the formula -(R,-O),,R where R, is a hydrocarbonradical containing two or three carbon atoms, R; is a hydrogen atom or ahydrocarbon radical containing one to five carbon atoms and n is aninteger of l, 2 or 3.

6. The process of claim 5, wherein the phosphoric acid ester is selectedfrom the group consisting of the tributyl ester of orthophosphoric acid,the triisobutyl ester of orthophosphoric acid, the tricresyl ester oforthophosphoric acid, the tricyclohexyl ester of orthophosphoric acid,monocyclohexyl di-n propyl orthophosphate and tri (butoxy-2 ethyl)phosphate.

7. The process of claim 1, wherein the alkanolamine is of the generalformula wherein at least one of the R groups is a group of the formula-CpH pOH where p is an integer from 2 to 6, and the other R groups areselected from the group consisting of a hydrogen atom, an alkyl groupcontaining one to 18 carbon atoms, and C, H OH group where r is aninteger from 2 to 6, the CpH p and C,H,, groups also capable ofcontaining an internal O- bond between two carbon atoms.

8. The process of claim 7, wherein the alkanolamines are selected fromthe group consisting of N-methyl diethanolamine, diethanolamine,diisopropanolamine, monoethanolamine, di-n propanolamine,triethanolamine, N- butyl diethanolamine, N-methyl diisopropanolamineand aminoethoxyethanol.

2. A process according to claim 1, wherein the reaction is carried outin two steps wherein the first one consists of contacting the impure gaswith the mixture of an alkanolamine and a phosphoric acid ester, and thesecond one consists of treating the resulting solution, separated fromthe purified gas, with the molecular oxygen-containing gas.
 3. A processaccording to claim 1, wherein the alkanolamine is used in an amount ofabout 0.1 to 50 percent with respect to the weight of the mixture of thephosphoric acid ester and the alkanolamine.
 4. A process according toclaim 1, wherein the step of contacting the gas with phosphoric acidester and the step of oxidizing the resulting solution are carried outsimultaneously.
 5. The process of claim 1, wherein the phosphoric acidester is of the general formula PO(OR)3 wherein R is selected from thegroup consisting of a monovalent hydrocarbon radical containing aboutthree to 20 carbon atoms and a radical of the formula -(R1-O)n-R2 whereR1 is a hydrocarbon radical containing two or three carbon atoms, R2 isa hydrogen atom or a hydrocarbon radical containing one to five carbonatoms and n is an integer of 1, 2 or
 3. 6. The process of claim 5,wherein the phosphoric acid ester is selected from the group consistingof the tributyl ester of orthophosphoric acid, the triisobutyl ester oforthophosphoric acid, the tricresyl ester of orthophosphoric acid, thetricyclohexyl ester of orthophosphoric acid, monocyclohexyl di-n propylorthophosphate and tri (butoxy-2 ethyl) phosphate.
 7. The process ofclaim 1, wherein the alkanolamine is of the general formula wherein atleast one of the R groups is a group of the formula -CpH2p-OH where p isan integer from 2 to 6, and the other R groups are selected from thegroup consisting of a hydrogen atom, an alkyl group containing one to 18carbon atoms, and -Cr H2r -OH group where r is an integer from 2 to 6,the CpH2p and CrH2r groups also capable of containing an internal -O-bond between two carbon atoms.
 8. The process of claim 7, wherein thealkanolamines are selected from the group consisting of N-methyldiethanolamine, diethanolamine, diisopropanolamine, monoethanolamine,di-n propanolamine, triethanolamine, N-butyl diethanolamine, N-methyldiisopropanolamine and aminoethoxyethanol.